1. Field of the Invention
The present invention relates to the development of underground deposits, such as hydrocarbon deposits comprising a fracture network.
2. Description of the Prior Art
The petroleum industry, and more precisely petroleum deposit exploration and development, require that knowledge of the underground geology is as perfect as possible to efficiently provide an evaluation of reserves, a modeling of production or development management. In fact, determining the location of a production well or an injection well, the composition of the drilling mud, the completion characteristics, choosing a method of recovering hydrocarbons (such as water injection for example) and the parameters necessary for the implementation of this method (such as injection pressure, production rate, etc.) require good knowledge of the deposit. Knowledge of the deposit notably means knowing the petrophysical properties of the subsoil at any point in space, and being able to predict the flows likely to occur there.
To do this, for some time the petroleum industry has combined field (in situ) measurements with experimental modeling (performed in the laboratory) and/or numerical modeling (using software). Petroleum deposit modeling thus constitutes a technical step essential to any deposit exploration or development. The object of this modeling is to provide a description of the deposit.
Fractured reservoirs constitute an extreme type of heterogeneous reservoirs comprising two contrasting media, a matrix medium containing the major part of the oil in place and displaying low permeability, and a fractured medium representing less than 1% of the oil in place and highly conductive. The fractured medium itself may be complex, with different sets of fractures characterized by their respective density, length, orientation, inclination and opening. The fractured medium is all the fractures. The matrix medium is all the rock around the fractured medium.
The development of fractured reservoirs requires knowledge of the role of the fractures which is perfect as possible. What is referred to as a “fracture” is a plane discontinuity of very small thickness in relation to its extent, representing a rupture plane in a rock of the deposit. On the one hand, knowledge of the distribution and of the behavior of these fractures can be used to optimize the location and the spacing between wells to be drilled through the oil-bearing deposit. On the other hand, the geometry of the fracture network conditions fluid displacement, both at the scale of the reservoir and at the local scale where it determines elementary matrix blocks in which the oil is trapped. Knowing the distribution of the fractures is therefore also very useful, at a later stage, for reservoir engineers seeking to calibrate the models they construct for simulating deposits in order to reproduce or to predict past or future production curves. For these purposes, geoscientists have three-dimensional images of deposits, for locating a large number of fractures.
Thus, to reproduce or predict (i.e. “simulate”) the production of hydrocarbons on starting production from a deposit according to a given production scenario (characterized by the position of the wells, the recovery method, etc.), reservoir engineers use calculation software, called a “reservoir simulator” (or “flow simulator”), which calculates the flows and the changes in pressure within the reservoir represented by the reservoir model (reservoir image). The results of these calculations enable them to predict and optimize the deposit in terms of flow rate and/or amount of hydrocarbons that are recovered. Calculating the behavior of the reservoir according to a given production scenario constitutes a “reservoir simulation.”
For carrying out simulations of flow around a well or at the scale of a few reservoir grid cells (˜km2), taking into account a geologically representative discrete fracture network, it is necessary to construct a grid of the matrix medium (rock) and a grid of the fractured medium. The latter must be adapted to the geometric (location of diffuse faults and fractures in three dimensions) and dynamic heterogeneities, since the fractured medium is often much more conductive of fluids than the matrix medium. These simulation zones, if they are fractured, may have up to a million fractures, with sizes varying from one to several hundred meters, and with very variable geometries: dip, strike, shape.
This step is very useful for the hydrodynamic calibration of fracture models. In fact, the discontinuity of hydraulic properties (dominant permeability in the fractures and storage capacity in the matrix) often leads to using the double-medium approach (homogenized properties) on reservoir models (hectometric grid cell). These models are based on the assumption that the representative volume element (RVE) is attained in the grid cell and the fracturing of the medium is sufficiently large to be able to apply homogenization methods (stochastic periodicity of fracturing is used, for example).
As part of the development of petroleum deposits, discrete flow simulation methods are used, notably, for the scaling of permeabilities (at the scale of a reservoir cell) and for dynamic tests (a zone of interest (ZOI) of small size compared to that of the reservoir). The calculation times appear to be crucial since the calculation is often performed sequentially and many times in optimization loops. It is known that in the case of dense fracturing (very connected fractures), analytical methods are applicable while in the case of a low connectivity index, the numerical simulation on a discrete fracture network (DFN) is necessary.
The numerical model, i.e. the matrix of transmissivities relating to the various objects (diffuse faults, grid cells of the matrix medium) must respect a number of criteria:                It must be applicable to a large number of fractures (several thousand fracture nodes);        It must restore the connectivity of the fracture network;        It must be adaptable in order to take into account all fracture models (several scales of fracturing, 3D disordered fractures, tight faults, dynamic properties over time, etc.);        It must be the shape of the fractures (any plane convex polygons or ellipses) and the heterogeneities of intersection between 3D fractures must be taken into account in the plane gridding of each of the fractures;        The number of simulation nodes must be restricted for being able to use a numerical solver;        It must be quick and inexpensive in memory (usable on a normal workstation and not just on a supercomputer).        
With such requirements, conventional (finite element or finite volume) grids and the methods derived from them for a local construction of transmissivities, are not applicable due to an excessively high number of nodes necessary for the simulation.
The known technique implemented in the FracaFlow™ software (IFP Energies nouvelles, France) which overcomes these limitations by using a physical approach based on analytical solutions of plane flow problems. The fractures here are assumed to be constrained by geological beds (they cross them entirely) and to be subvertical. A fracture is considered constrained to the beds if it breaks off on changes of geological bed. These assumptions ensure that all intersections take place on any intermediate plane parallel to the geological layers. In the midplane of each geological bed, the nodes are placed on the intersections (a point) of the fractures on the plane (a matrix node and a fracture node at the same place). The vertical connections are borne by the fractures crossing several layers and the volumes associated with the nodes are calculated as the set of points (on the 2D plane, propagated vertically to the thickness of the layer) closest to the node (in the medium considered).
This method reaches its limits when the fractures are not constrained to the beds and/or the dip of the fractures is not vertical. The intersections are consequently not present on each intermediate plane and the vertical connectivity cannot be honored. By increasing the number of intermediate planes, the error can be reduced (without ever being correct in the case of subhorizontal fractures) but the number of nodes increases significantly and exceeds the limits imposed by the solvers.